1. Field of the Invention
This invention relates to an optical type encoder, and in particular to an optical type encoder in which a light is projected onto an optical type scale on which a diffraction grating is formed and interference fringes obtained by causing a plurality of diffracted lights created by the optical type scale to interfere with one another are photoelectrically converted to thereby measure the displacement of the optical type scale.
2. Related Background Art
In recent years, in precision machines such as NC machine tools and semiconductor printing apparatuses, there has been required a precise measuring machine which can measure the displacement of a movable member such as a stage in the unit of 1 .mu.m or less (submicron).
In Japanese Laid-Open Pat. Application No. 62-193922 and Japanese Laid-Open Pat. Application No. 62-193923 filed by the Applicant, there are proposed optical type encoders using a coherent light beam and a diffraction grating to detect the displacement of an optical type scale highly accurately.
In the apparatuses of these publications, a laser beam from a laser is divided into a plurality of light beams, and these light beams are caused to enter the diffraction grating of an optical type scale connected to a rotating object or a rectilinearly moving object, and interference fringes obtained by causing a plurality of diffracted lights created by said diffraction grating to interfere with one another are photoelectrically detected by photoelectric converting means, thereby finding the displacement of the optical type scale, i.e., the displacement of the moving object, highly accurately.
In this case, it is important for accomplishing highly accurate measurement to construct the apparatus so that the amplitude of the output signal from the photoelectric converting means may always be constant, but since the output value from the laser (the intensity of the laser light) fluctuates due to the manufacturing error of the diffraction grating of the optical type scale or to a change in the environment around the laser, it has been difficult to obtain an output signal of a predetermined amplitude from the photoelectric converting means. Accordingly, with the prior-art optical type encoder, the displacement of the optical type scale could not be measured highly accurately.